The field is internal combustion engines. Particularly, the field includes opposed-piston engines. In more particular applications, the field relates to a cylinder equipped with opposed pistons in which the cylinder bore has a construction providing solid lubrication of the bore/piston surface interfaces in the top ring reversal zones of the cylinder bore surface.
When compared with four-stroke engines, two-stroke opposed-piston engines have acknowledged advantages of specific output, power density, and power-to-weight ratio. For these and other reasons, after almost a century of limited use, increasing attention is being given to the utilization of opposed-piston engines in a wide variety of modern transportation applications.
A representative opposed-piston engine is illustrated in FIGS. 1 and 2. The opposed-piston engine includes one or more cylinders 10, each with a bore 12 and longitudinally-displaced exhaust and intake ports 14 and 16 machined or formed therein. Each of one or more fuel injector nozzles 17 is located in a respective hole that opens through the side of the cylinder; in some aspects, the holes are at or near the longitudinal center of the cylinder. Two pistons 20, 22 are disposed in the bore 12 with their end surfaces 20e, 22e in opposition to each other. For convenience, the piston 20 is referred as the “exhaust” piston because of its proximity to the exhaust port 14, and the end of the cylinder wherein the exhaust port is formed is referred to as the “exhaust end”. Similarly, the piston 22 is referred as the “intake” piston because of its proximity to the intake port 16, and the corresponding end of the cylinder is the “intake end”. One or more rings 23 are mounted in circumferential grooves formed in each of the pistons 20, 22 near the piston's crown.
The exhaust and intake ports 14 and 16 of the cylinder 10 seen in FIG. 1 are similarly although not necessarily identically constructed. Consequently, although only the intake port construction is visible in the figure, the following explanation pertains to the exhaust port as well. As per FIG. 1, the intake port 16 includes at least one sequence of openings 28 through the sidewall and in a peripheral direction of the cylinder 10 near the intake end of the cylinder. For example, the openings 28 extend in a circumferential direction. The openings 28 are separated by bridges 29 (sometimes called “bars”). Relatedly, the term “port” in the description to follow refers to an alternating series of openings and bridges peripherally spaced around the cylinder near one of its ends. In some descriptions the openings themselves are called ports; however, the construction of one or more peripheral sequences of such “ports” is no different than the port constructions shown in the figures to be discussed.
Operation of an opposed-piston engine with one or more cylinders 10 is well understood. With reference to FIG. 2, in response to combustion occurring between the end surfaces 20e, 22e the opposed pistons move away from respective top dead center (TDC) positions where they are at their closest positions relative to one another in the cylinder. While moving from TDC, the pistons keep their associated ports closed until they approach respective bottom dead center (BDC) positions in which they are furthest apart from each other. In a useful, but not a necessary aspect of opposed-piston engine operation, a phase offset can be introduced in the piston movements around their BDC positions so as to produce a sequence in which the exhaust port 14 opens as the exhaust piston 20 moves toward BDC while the intake port 16 is still closed so that exhaust gasses produced by combustion start to flow out of the exhaust port 14. As the pistons continue moving away from each other, the intake port 16 opens while the exhaust port 14 is still open and a charge of pressurized air (“charge air”), with or without recirculated exhaust gas, is forced into the cylinder 10. The charge air entering the cylinder drives exhaust gasses produced by combustion out of the exhaust port 14.
As per FIG. 1, presuming the phase offset mentioned above, the exhaust port 14 closes first, after the pistons reverse direction and begin moving toward TDC. The intake port 16 then closes and the charge air in the cylinder is compressed between the end surfaces 20e and 22e. As best seen in FIG. 2, as the pistons advance toward their respective TDC locations in the cylinder bore, fuel 40 (typically, but not necessarily, diesel) is injected through nozzles 17 directly into the charge air in the bore 12, between the end surfaces 20e, 22e of the pistons. The mixture of charge air and fuel is compressed in a combustion chamber 32 defined between the end surfaces 20e and 22e when the pistons 20 and 22 are near their respective TDC locations. When the mixture reaches an ignition temperature, the fuel ignites in the combustion chamber, driving the pistons apart toward their respective BDC locations.
Preferably, although not necessarily, the construction of an opposed-piston engine cylinder includes a cylinder liner, which is a cylindrical sleeve fitted into an engine block or frame to form the cylinder. The liner can be a single piece, or it can be assembled from two or more pieces. In this regard, a single-piece liner is described in US 2010/0212613 A1 and a multi-piece liner is described in U.S. application Ser. No. 13/136,402. A cylinder liner 110 is illustrated in FIG. 3; the internal surface 113 of the liner defines the cylinder bore.
In order to increase the mechanical effectiveness and durability of an opposed-piston engine, it is desirable to reduce energy loss and wear caused by friction between the cylinder bore and the opposed pistons disposed for sliding movement therein. In the opposed-piston context illustrated in FIG. 3, areas of the cylinder bore 113 in which friction between the bore and the piston rings is particularly severe include: 1) top reversal zones 132 where the pistons reach TDC, 2) bottom reversal zones 135 where the pistons reach BDC, and 3) the port bridges 127, 129. The reversal zones are those annular sectors of the cylinder bore surface in which the pistons change direction and the reciprocating motion of the rings' sliding velocity is at zero.
At top ring reversal in any engine, sliding velocity of a piston is low while the pressure bearing the compression ring against the bore of the cylinder is high, which tends to cause a boundary lubrication condition that is metal-on-metal contact with no liquid lubricant separating the solid bodies. This boundary condition is termed dry lubrication. Movement of the piston in the bore under conditions of dry lubrication can damage the bore and/or the rings. Other undesirable effects of dry lubrication include increased friction and wear.
Current solutions employed to mitigate undesirable effects associated with dry lubrication include use of solid lubricants in the bore/piston interface. For example, grey cast iron material can be used to form the cylinder bore, which implies carbon content >2.1% and also the presence of graphite in a lamellar structure. The flake (lamellar) graphite acts as a solid lubricant that is effective in preventing scuffing and minimizing friction. However, grey iron is weak in tension, compared to steel, because the graphite flakes act as crack initiation sites and the structure does not resist crack propagation, which reduces fatigue strength of the grey cast iron material.
As per FIG. 4, a unique characteristic of opposed-piston engines is a requirement for at least one hole 30 through each cylinder to accommodate a fuel injector positioned between the exhaust and intake ports. In some aspects, more than one injector hole is desirable; in other aspects, additional holes may be desirable for operations such as engine braking. These holes interrupt the hoop structure of a cylinder liner 32; if the liner is made of grey iron, such holes lead to concentrated zones of tensile stress and possible lateral cracking 34 due to tensile fatigue. It therefore would be advantageous to provide a construction for a cylinder liner of an opposed-piston engine that would overcome this tensile stress condition while still reducing the effects of friction between the piston rings and the cylinder bore in the top ring reversal zones.